Permanent crowns, permanent facings and permanent bite caps have been used for many years to replace damaged or decayed teeth. They are also used to improve the aesthetics of teeth, even in cases where there is no damage or decay. The restorative process involves a number of steps. During the restorative process, a dentist must remove the damaged or decayed portion of the tooth, protect the remaining portion of the natural tooth (in the most severe case, a tooth stub) with a temporary (a “temporary” being one of a temporary crown, temporary facing or temporary bite cap) and then later, after a permanent crown, facing or bite cap has been made, restore the tooth by applying the permanent structure to the remaining portion of the natural tooth.
For the purposes of this specification, the remaining portion of the natural tooth will be referred to as the tooth when the temporization process involves an onlay or a veneer and when the temporization process involves a crown form, the remaining portion of the natural tooth will be referred to as a tooth stub. For the purposes of this specification, a “temporary bite cap” and a “temporary facing” are temporaries that cover less of the tooth than does a temporary crown. A temporary bite cap covers at least one cusp of the tooth and a temporary facing covers at least one side of a tooth. A “temporary”, therefore, refers collectively to temporary crowns, and variants thereof, including temporary bite caps and temporary facings. A “provisional” refers collectively to crown forms, onlays, veneers and variants thereof. Temporaries, therefore, are further defined as the combination of a provisional, a resin filler and an optional temporary or permanent adhesive.
When a permanent crown is required, the temporization process involves fitting a temporary crown over the tooth stub, affixing the temporary crown, and then at a later time, removing the temporary crown in order to place the permanent crown on the stub. The temporary crown protects the stub and functions, with the stub, as a complete tooth while the permanent crown is being made.
When a permanent facing or a permanent bite cap is required, the temporization process involves fitting the tooth with a temporary facing or a temporary bite cap, affixing these temporaries, and then, as above, removing the temporary structure in order to place the permanent structure on the tooth.
Whether the dentist decides to use a crown form, a veneer or an onlay in the temporization process depends on the extent of damage to the tooth. When a large amount of tooth has been damaged, the tooth will be reduced to a stub and a crown form will be required. If the damage is restricted to the occlusal surface of the tooth, an onlay will be required, and if the damage has been restricted to the side of the tooth, a veneer will be required. As would be apparent, there is a broad range in the amount of tooth that is removed from case to case, and correspondingly, there is a broad range of tooth coverage provided by the various temporaries.
Having regard to the foregoing, there is no clear conceptual distinction between the various temporaries, a large veneer being essentially the same as a small crown form (for example, a ¾ crown form), and similarly, a large onlay being essentially the same as a small crown form (for example a ¾ crown form). For example, an onlay may comprise as little as one cusp and one wall and as much as the entire occlusal surface and up to ¾ of each wall. Once the temporization is comprised of more than ¾ of the walls, it is no longer considered to be an onlay, but rather, it is often referred to as a ¾ crown form. Similarly, a veneer may comprise as little as one wall. Once the temporary comprises the incisal or occlusal surface and, for example, the lingual wall and enough of the neighbour walls to extend past the narrowest part of the space between the temporary and the adjacent structure, it is usually referred to as a ¾ crown form. As would be apparent, the relationship between crown forms and veneers and onlays extends to the resultant temporary crowns, facings and bite caps.
Crown forms are used extensively in the temporization process. Until recently, these forms are typically made of stainless steel, aluminum, anodized gold, polycarbonate or clear plastic such as 3M™ clear strip forms. Many different sizes and shapes are required. For example, crown forms are sized for deciduous and permanent teeth, bicuspids, molars, canines and incisors. Accordingly, a dentist must have a large collection of crown forms readily available when doing restorative work.
In contemporary dental practice, the crown form is filled with resin and the resin is affixed non-permanently to the stub of the tooth by means of a cement, or by photo-active temporary adhesives. The resins are commonly heterogeneous materials having three essential phases: (1) a polymeric matrix which comprises the continuous phase; (2) fillers of various types, sizes, shapes and morphologies that constitute the disperse phase; and (3) an interfacial phase that, in varying degrees, bonds the continuous and disperse phases into a unitary material rather than a simple admixture
It is the crown form in combination with the resin and cement that constitutes the temporary crown known previously.
The resins with which the crown form is filled typically contain predominately methacrylates, diacrylates or dimethacrylates as the polymeric matrix, as disclosed for example in U.S. Pat. No. 6,114,408, herein incorporated by reference, and additionally have a photochemical initiator, such as camphorquinone, that is either mixed with the polymorphic matrix just prior to use, or is provided in a photo-active temporary adhesive formulation. Once the temporary adhesive, resin and crown form are placed on the tooth stub, ultraviolet or visible light is used to activate the initiator, which then produces free radicals. These free radicals in turn initiate polymerization. The polymerization process is exothermic, and therefore can be irritating to the patient unless great care is taken.
For the purposed of this specification, resin filler and provisional materials that can cross-polymerize with one another, such as those listed above, or can otherwise suitably bond together, are considered to be compatible. If this polymerization can be initiated at the same time in the provisional and the resin filler, or alternatively, initiated in one and the free radicals arising therefrom then initiating polymerization in the other, then the resin filler and the provisional are considered to be cooperatively curable.
Many of the cements and temporary adhesives commonly used in the temporization process do not chemically adhere to the tooth stub, nor do they chemically adhere to the crown form. Rather, the crown form is retained on the resin and cement or temporary adhesive by physical means (at least friction and/or surface adhesion, and preferably interference) and similarly, the resin and cement or temporary adhesive are retained on the stub by physical means (friction and surface adhesion).
On the other hand, the temporary adhesives that make use of polyacrylic acid, such as zinc polycarboxylate and glass polyalkenoate (glass ionomer), provide a carboxylate group that chemically adheres to the dentin of the tooth stub (see for example the information on this subject that is recorded in http://www.cda-adc.ca/jcda/vol-64/issue-8/569.html, herein incorporated by reference). However, these temporary adhesives do not adhere to the crown form. Hence, the crown form is again held in place by physical rather than chemical means.
The crown form has to be fitted carefully in the patient's mouth as there is little room for adjustment after the temporary crown is affixed. This care is in part required because perforce the temporary crown is not firmly attached to the stub (the dentist must be able to remove it before placing the permanent crown on the stub), and, therefore, can be easily dislodged. Further, the crown form material is thin, and hence, if the temporary crown is, for example, seated proud of the other teeth, the dentist when grinding the temporary crown for bite adjustment can remove only a limited amount of material from the surface before grinding through the crown form and into the resin. Grinding through the crown form can result in distortion of the crown form, and in the worst-case scenario, failure of the temporary crown.
Recently, sculptable composite resins have been developed to overcome many of the deficiencies in the prior temporary crown technology. Products such as that sold under REVOTEK LC™ as described, for example, in http://www.gcamerica.com/gcreveotk.htlm, herein incorporated by reference, are comprised of urethane dimethylacrylate, a color filler and a photochemical initiator, and are typically provided in the form of a putty. Once the tooth stub is prepared, the putty is used to sculpt a temporary crown in situ, photo-cured briefly to stabilize the shape, and removed from the patient's mouth for final curing. The sculpted temporary crown is then affixed to the patient's tooth stub using cement or photo-active temporary adhesives. In this procedure, no crown form is used.
While the sculptable temporary crown technology improves upon the prior art, it still requires that the patient spend considerable time in the dentist's chair during the temporization process. Further, it is essential to use the services of a highly skilled technician to sculpt the temporary crown in situ.